This disclosure is directed to systems and methods for maintaining constant or desired hue trajectory across a range of tint variations of spot colors or for rendering a target color lying between a pair of spot colors according to a desired color trajectory between the two spot colors, within images formed in an image forming device.
Color images are commonly represented in image forming systems as a series of discrete separations. Each separation comprises a set of color signals for one primary color. Color images prepared for printing typically comprise four primary color signals, i.e., cyan (C), magenta (M), and yellow (Y), and black (K) signals, each signal corresponding to a particular process ink of that color. Together, these CMYK signals may be considered the colorant signals in a particular image forming device. Because the use of CMYK is well understood in the art, further explanation regarding its use and function in rendering images need not be articulated here. Furthermore, some image forming systems enable the use of more than 4 process inks, thereby producing a larger gamut, or range of colors. For the rest of the disclosure, we will use the case of CMYK primaries to teach the concepts, with the understanding that the ideas extend in a straightforward fashion to extended colorant sets.
Additionally, the use of spot colors is also well known in the art. These spot colors refer to accepted, and identified, shades of color, and have traditionally been implemented using special inks which are unique to the particular spot color. One of the best known examples of spot colors is the Pantone color scheme. However, because the use of true spot colors is expensive and inconvenient, for many applications several methods for emulating spot colors in image forming systems exist. Such emulation can be carried out using the CMYK process inks, or where the image forming systems allow, additional process inks may be used to more closely emulate the spot colors. Although for simplicity spot color emulation is discussed in this document primarily in terms of emulation using CMYK process inks, it is to be understood that all forms of emulation, including the use of more than four process inks, is likewise covered.
One widely known conventional method for spot color emulation, and/or spot color rendering, in image forming devices is basically understood as follows. In this method, spot colors can be described in the applicable page description language (“PDL”) such as PostScript, or PDF. In addition, tints of the spot colors typically can be described in the PDL. During raster image processing (“RIP”) in the digital front end (“DFE”), spot colors are detected, and a predetermined CMYK recipe for rendering the spot color is retrieved from generally a pre-stored table of values for each corresponding spot color.
The emulation of spot colors using CMYK recipes, or even recipes involving more than 4 process inks, is fairly well defined when printing the spot color with a full (100%) tint. However, printing tints of the spot colors using this method is typically not well defined. In some previously used methods, when printing a tint of 40% for a spot color, the tint would be emulated by likewise using a 40% recipe of each of the CMYK colors in the spot color emulation recipe. This would often result in inappropriate representations of the true tint of the spot color. In particular, the gradation of tints over a full sweep would often be inconsistent. In other words, the hue of the tint, or of a sweep formed by a series of tints, would be inconsistent.
In an effort to improve the accuracy of tint rendition of spot colors in sweeps an alternate method was used in which, instead of retrieving predetermined CMYK values directly from a pre-stored table, a device independent target is retrieved in a color space representative of the spot color and converted to a CMYK value for rendering the spot color image.
As described, for example, in chapter 1 of the Digital Color Imaging Handbook, edited by Gaurav Sharma (CRC Press, 2003) and incorporated herein by reference, color may be measured in terms of three components which are often referred to as L*, a* and b*, which define parameters by which any color may be specified in the coordinate system. In an L*a*b* system, L* generally corresponds to a lightness-darkness scale value, a* generally corresponds to a red-green scale value, and b* generally corresponds to a yellow-blue scale value. Appropriate L*a*b* values, for emulating spot color, are looked up from a table, and are converted to CMYK values to achieve the final CMYK recipe. The L*a*b* values are device independent, and the CMYK values are device dependent. This conversion process utilizes a color transformation mechanism in the RIP.
To emulate the tints in the sweep various methods have been applied to the L*a*b* values of the solid spot color, referred to herein as the base set of color space values. For example, linear interpolation was used as a method. The use of a color space system, such as the L*a*b* system results in a more perceptually constant hue over the sweep. However, discontinuities would often be evident between the final tint approaching the true spot color and the spot color itself, due to unavoidable discrepancies between the recipe calculated from the target L*a*b* color and the recipe in the spot color emulation table. Additionally, many spot color emulation systems allow a user to personalize the CMYK recipe used to emulate the spot color. Altered CMYK recipes would result in even more discontinuity between the spot color emulation, and the tints derived from the independent, and unaltered L*a*b* value tints.